Two types of magnetic nondestructive systems are in widespread use currently, namely, the "eddy-current" system and the "flux leakage" system. In the former system, probes adjacent a test object are excited and apply a magnetic field thereto which induces eddy-currents in the object. In the latter system, probes adjacent the test object measure flux leakage therefrom. In both systems, the probe output signals vary in characteristics when a flaw in the specimen is encountered and associated analyzing circuitry receives the probe output signals and provides system output indication of object faults.
Both of the described systems are susceptible to variation in probe output signal magnitude as spacing between the probe and the specimen varies. For a given probe and flaw, applicants herein have observed that the ratio of probe output signal amplitudes over a range of one hundred and fifty mils may vary by as much as a factor of fifty. Thus, a flaw measurement signal at the closest location to the object in such range may have an amplitude of fifty times that of the measurement signal at a location spaced one hundred and fifty mils from such closest location. Evidently, in the absence of some compensation for this signal strength variation, flaws present in an object can go undetected at probe spacings distal therefrom.
Constancy of probe spacing in relation to the object is fully implausible as a solution to the problem. Thus, an object may exhibit surface depth irregularity in substantial measure, whereby a fixedly positioned probe would experience a large spacing range. Further, it is typical that measurements are made with relative motion as between the object and the probes in which case spacing variation occurs even where a specimen might itself have surface depth variation of quite low magnitude.
Known efforts to solve this problem are considered to be empirical or at least not as analytical as would be desirable to fully address a solution. Typically, the prior art has addressed the problem with an outset recognition that the probe output signal must somehow be attenuated greatly at close location in the spacing range, with attenuation then being successively less as the outside location of the spacing range is reached. One practice in this respect is seen in U.S. Pat. No. 3,611,120, wherein the probe includes an L-C circuit which is designed to exhibit resonance at the outside location of the spacing range and to be loaded inductively by the object as the probe approaches the specimen. Gain through the probe is thus lessened as the departure from resonance increases by probe loading by the object.
Perhaps a better understanding of the reach of this type of compensation is seen from commercially-available eddy-current type distance sensors of the Electro Corporation, one of which is discussed below for use in the subject invention. Such device is of the resonance at maximum spacing variety, employing variation in the "Q" of the device to provide output indication of spacing. Literature discussion of this device indicates that quite close linearity is obtainable and is precisely indicative of probe spacing as induced eddy-currents sap field energy in a linear manner.
Attainment of linearity of attenuation with distance is seen by applicants to be an incomplete solution to the problem at hand, since they note, as is fully developed below, that spacing or distance is but one input to the solution of such problem.